1. Field of the Disclosure
This disclosure relates to a liquid crystal display device, and more particularly to the liquid crystal display device of an in-plane-switching mode adapted to prevent a residual image problem.
2. Description of the Related Art
Nowadays, liquid crystal display (LCD) devices have been highlighted as the next generation of display devices. This results from the fact that the LCD devices have features of low power consumption, superior portability, intensive technology, highly added value, and others. The LCD devices are configured to use light transmittance varying along the molecular alignment of a liquid crystal, in order to provide images to users.
Among the LCD devices, a twisted nematic (TN) mode LCD device has been mainly used because of its simple configuration and easy formation. The TN mode LCD device forces liquid crystal molecules to be aligned from a horizontal direction into a perpendicular direction against a substrate when a voltage is applied to it. The liquid crystal molecule allows light transmittance to vary along a viewing angle (or a viewing direction) due to its reflective index anisotropy. As such, the TN mode LCD device driving the liquid crystal molecules has a narrowed viewing angle.
In order to enhance the viewing angle, an in-plane-switching (IPS) mode LCD device has been proposed. The IPS mode LCD device includes first and second substrates opposite each other and a liquid crystal layer interposed between the first and second substrates. The first substrate includes a thin film transistor, a pixel electrode with a plurality of branched sub-pixel electrodes, a common electrode with a plurality of branched sub-common electrodes, which are formed in each pixel region on it. The sub-pixel electrodes and the sub-common electrodes are arranged alternately with each other and used to form a horizontal electric field. The second substrate is configured to include a black matrix, color filter patterns, an overcoat layer, and an alignment film on it. Such an IPS mode LCD device forces the liquid crystal molecules to align parallel to the horizontal electric field. Accordingly, the viewing angle can be enhanced.
The alignment film formed from an organic material on at least one of the first and second substrates can discharge ionized and polarized impurities under high temperature. The impurities can be absorbed and adhered between the alignment film and a lower film. As such, a parasitic capacitance can be generated in the interfacial surface of the alignment film and the lower layer. The parasitic capacitance varies an effective voltage applied to the liquid crystal layer, thereby causing the appearance of a residual image.
More specifically, if an image is displayed for an extended period of time, the parasitic capacitor charges a direct current (DC) voltage applied for the image display. The charged DC voltage causes molecules within the liquid crystal layer to realign even when an additional DC voltage is not applied to the liquid crystal layer. Accordingly, although a new DC voltage for changing the image is applied to the liquid crystal layer, a trace of the image formed by the charged DC voltage remains. In other words, a residual image appears due to the DC voltage charged in the parasitic capacitor.